The present invention relates to the casting of metal parts and the like. More specifically, the invention relates to an exothermic formed body constructed for use as a riser in an upper portion of a mold.
The invention is specifically applicable to a segmented riser having a spherical internal cavity. The riser is formed from a material having specific exothermic properties. However, it will be recognized that the exothermic material which is described herein can be used for purposes other than in a riser for ingot molding.
After refining operations are completed in the process of manufacturing various metals, such as steel, the molten metal is teemed into a mold where the metal is allowed to solidify into desired cast bodies or ingots. As the metal cools in the mold, imperfections are created in the metal due to the shrinkage thereof during cooling. Such imperfections are caused by the metal in the body or ingot cooling from its exterior periphery inwardly in a gradual manner. During cooling and solidification of the casting, a thin skin of frozen metal forms around the outer part of the mold cavity. This skin forms a rigid shell which acts as a mold for the remainder of the casting. One common imperfection is a funnel shaped contraction in the head of the ingot known as "pipe." Another common imperfection is gas bubbles entrapped in the metal during the cooling process. Other imperfections encountered in the cooling of cast metal include center segregation and porosity, sometimes called "fish tails." All of these reduce the strength of the metal.
The formation of such imperfections due to metal shrinkage in cast metal bodies has been found to be preventable by the application of a riser or hot top which retains and feeds molten metal to the shrinking ingot before and during solidification. A hot top can consist of clay molds, ceramic sleeves within refractory casings or, more commonly, a metal casing with an interior layer of an insulating material bonded to the casing. The hot top is meant to absorb heat from the molten metal less rapidly than the walls of the mold due to the use of insulating material for the hot top walls. Thereby an overlying pool of molten steel is meant to be furnished which feeds metal down into the ingot to overcome the shrinkage problems due to solidification of the metal in the ingot. When risers are small, they cool fast and therefore an exothermic material is sometimes used instead of insulating material on one or more walls of the riser to prolong the time during which the metal remains molten by heating the metal contained in the riser.
One problem with conventional hot tops or risers is that they are generally square in cross section and, therefore, do not minimize the heat loss of the molten metal in the riser structure by reducing the surface area of the molten metal which is exposed to the riser structure.
Another problem with conventional riser assemblies or hot tops is that even when exothermic materials are used, the exothermic properties of the riser structure material are not suitable for prolonging the time during which the metal remains molten so as to adequately fulfill the requirements of the steel molding process.
Even those riser constructions which have a suitable conformation, preferably a spherical conformation, for minimizing the heat loss of molten metal by reducing the surface area of the metal exposed to the riser structure, are not made of a suitable exothermic material which would prolong the heat available to the riser in order to keep the metal therein in a molten state for a sufficient length of time while the rest of the mold cools.
Additionally, when the conventional exothermic material has burned, the resulting clinker does not act as a suitable insulation layer to keep heat in the molten metal held in the riser cavity.
Accordingly, it has been considered desirable to develop a new and improved riser assembly which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.